High capacity, cost effective batteries are needed for a range of applications, including greenhouse gas reduction applications, overcoming the battery driven “range anxiety” of electric vehicles, and increased energy capacity storage for the electric grid.
A zirconia stabilized VB2 air battery has been described. See, e.g., S. Licht et al., Chem. Comm., 3257-3259, 2008; S. Licht et al., Electrochem. Solid State Lett. 14, A83-A85, 2011; and S. Licht et al., Electrochem. Solid State Lett. 15, A12-A14, 2012. This 11e− (eleven electron) per molecule, room temperature, aqueous electrolyte battery has the highest volumetric energy capacity for a battery, with an intrinsic capacity greater than that of gasoline and an order of magnitude higher than that of conventional lithium ion batteries. The challenge, however, is recharging the battery (i.e., electrochemically reinserting 11e− into the battery discharge products).
Several classes of molten electrolyte batteries have been investigated. A molten sulfur battery has been studied, particularly for electric car and grid applications, See, e.g., B. Dunn et al., Science, 334, 9280935, 2011; and J. B. Goodenough et al., J. Solid State Electrochem., 16, 2019-2029, 2012. During discharge, the battery uses sulfur and sodium (or potassium) for the respective cathode and anode storage materials, and these high temperature molten components are kept from chemically reacting by a solid electrolyte beta alumina separator. Sulfur cells have moderately high capacity. Both the molten and room temperature class of sulfur cathode batteries are limited by the maximum intrinsic capacity of the 2e− per sulfur (2 Faraday/32 g sulfur).
Another class of molten metal electrolyte batteries utilizes an insoluble, dense, molten cathode during discharge situated below a (less dense) molten metal anode floating on a molten electrolyte. Unlike the molten sulfur battery, this class does not require a solid electrolyte separator; but to date has lower capacity. An example of this class of batteries is the magnesium-antimony battery with a molten halide electrolyte. See, e.g., D. J. Bradwell et al., J. Amer. Chem. Soc., 134, 1895-1897, 2012.
Iron has been widely explored for battery storage due to its availability as a resource and its capability for multiple electron charge transfer. In the early 1900s, Edison developed rechargeable batteries based on the discharge of an iron anode (in an aqueous electrolyte at room temperature) to form iron oxide. See, e.g., U.S. Pat. Nos. 678,722 and 692,507. Retention of an even a small fraction of the intrinsic anodic storage capacity of these batteries has been a challenge, and room temperature iron batteries continue to be explored today. The 3e− cathodic capacity of iron oxides has also been explored. See, e.g., S. Licht et al., Science, 285, 1039-1042, 1999; .S. Licht et al., Energies, 3, 960-972, 2010; and M. Farmand et al., Electrochem. Comm., 13, 909-912, 2011.
A molten carbonate electrolytic conversion of iron oxide to iron as a CO2-free synthesis alternative to the conventional greenhouse gas intensive industrial production of iron metal has been described. The high solubility of iron oxide in lithiated molten carbonate electrolytes was demonstrated to lead to the facile splitting of iron oxide to iron metal with the concurrent release of oxygen. See e.g., S. Licht et al., Advanced Materials, 47, 5592-5612, 2011 and International Publication No. WO 2011/140209.
There is, however, a need for new rechargeable batteries that have the capacity for multiple electrons stored per molecule, have high intrinsic electric energy storage capacities and can operate at lower temperatures.